Some components of combustion turbines operate at high temperatures, and thus may require thermal barrier coatings (TBCs). Conventional TBCs typically comprise a thin layer of zirconia or other ceramic material. In some applications, the coatings must be erosion resistant and also abradable. Turbine ring seal segments must withstand erosion and must also have tight tolerances on a radially inward sealing surface opposed the tips of rotating turbine blades. To minimize these tolerances, the sealing surface of ring segments may be made abradable in order to reduce damage to the turbine blades upon occasional brushing contact of blade tips with the sealing surface.
Improvements in gas turbine efficiency rely on breakthroughs in several key technologies as well as enhancements to a broad range of current technologies. One of the key issues is a need to tightly control rotating blade tip clearance. This requires that turbine ring segments are able to absorb mechanical rubbing by rotating blade tips.
For modern conventionally cooled and closed loop steam cooled turbine ring segments, a thick thermal barrier coating of about 0.1 inch on the ring segment surface is required for rubbing purposes. The latest advanced gas turbine has a hot spot gas temperature of over 1,500 degrees C. at the first stage ring segment. Under such heat, a TBC surface temperature of over 1,300 degrees C. is expected. Thus, a conventional abradable TBC is no longer applicable because conventional TBCs are typically limited to a maximum surface temperature of about 1,150 degrees C.
Friable graded insulation (FGI) materials are disclosed in U.S. Pat. No. 6,641,907 commonly owned by the present assignee. The effectiveness of FGI as an abradable refractory coating is based upon control of macroscopic porosity in the FGI to deliver acceptable abradability. Such a coating may consist of hollow ceramic spheres in a matrix of alumina or aluminum phosphate. To bond an FGI layer to a metal ring segment, an FGI-filled metallic honeycomb structure has been proposed in U.S. Pat. No. 6,846,574 commonly owned by the present assignee. In this technique a high temperature metal alloy honeycomb is brazed to the metallic substrate. The honeycomb, once oxidized prior to FGI application, serves as a mechanical anchor and compliant bond surface for an FGI filler, and provides increased surface area for bonding.
Further advances in high temperature abradable surfaces for gas turbine ring segment surfaces are desired.